Soil Erosion Hazard Under the Current and Potential Climate Change Induced Loss of Soil Organic Matter in the Upper Blue Nile (Abay) River Basin, Ethiopia

Abstract

This chapter assesses soil erosion hazard in the Upper Blue Nile (Abay) River Basin of Ethiopia, where the Grand Renaissance Dam is under construction, under the existing land resource use practices and climatic conditions as well as a scenario of future potential change in soil organic matter (SOM) concentration by the projected climate change. The Revised Universal Soil Loss Equation (RUSLE) model was used to estimate the soil loss rate. To capture potential effects of SOM loss by climate change on soil erodibility, specifically due to increase in temperatures, arbitrary scenarios of 20 % and 50 % reduction in SOM concentration were considered. The use of dekadal rainfall from 1,634 points representing a 10 × 10 km spatial resolution is the key element of this study. Estimates show that the antecedent mean annual soil loss for the Basin was 16 Mg ha−1 year−1. Scenario analysis of SOM reduction by 20 % and 50 % resulted in mean annual soil erosion rates of 17 Mg ha−1 year−1 and 19 Mg ha−1 year−1, respectively. The mean annual soil loss for the 50 % SOM reduction scenario exceeds the estimated maximum soil loss tolerance level of the country (18 Mg ha−1 year−1). Total soil loss from the Basin was estimated at 280 Tg year−1, (Tg = teragram = 1012 g = 1 million Mg), compared with 300 Tg year−1 and 332 Tg year−1 for the 20 % and 50 % SOM reduction, respectively. The northeastern, eastern and southern parts of the Basin (~25 % of the total area) are prone to very severe soil erosion risks (>30 Mg ha−1 year−1). The lowest soil erosion rate (<10 Mg ha−1 year−1) was observed in the southwestern, western and northwestern parts of the Basin. The sediment delivery ratio of the Basin was estimated at ~50 %. The sediments transported from the Basin are already affecting reservoirs and irrigation canals in the downstream countries of Sudan and Egypt, and will also adversely affect the Grand Ethiopian Renaissance Dam. Using the estimated soil erosion rates, the Basin was divided into priority categories for conservation intervention. Sub-basins prone to severe soil erosion risks are Beshilo, Welaka, North Gojjam, Jemma and Muger, and these need immediate attention for soil conservation and watershed management planning.